The measurement of the total organic carbon present in solids or liquids is a common parameter used in a variety of industries, such as semiconductor, pharmaceutical, food, and petrochemical. In the petrochemical industry, the total organic carbon of rock from an oil or natural gas reservoir is known as a parameter that is considered in evaluating the quality of the reservoir. This is especially true when evaluating the quality of an oil or gas reservoir located within a rock known as “shale.” For example, the total organic carbon of the shale is used to assess organic richness, original hydrocarbon potential, thermal maturity, gas content, and gas yields. To discover new sources of oil or natural gas, operators are looking for shale with sufficient total organic carbon and other evidence that amounts of gas had been formed from that total organic carbon in order to evaluate the interest and the quality of the reservoir. The technique used routinely to evaluate the quality of the shale is a standard tool for hydrocarbon exploration known as the Rock-Eval technique.
In soils and sediments such as drill cuttings, two basic forms of carbon compounds may be present: total inorganic carbon (TIC) and total organic carbon (TOC). The TIC includes carbon compounds from the carbonates (e.g., calcite, dolomite, siderite) present in the rock and/or soil, whereas the TOC comes from any organic matter (OM) present in the rock and/or soil. The organic matter is defined as a variety of materials ranging from simple molecules of various types such as sugars, complex proteins, to fats, waxes, or complex hydrocarbons.
Many methods exist to determine the TOC, but currently the method that utilizes dry combustion of carbon compounds is the most widely used. Many instruments are available in the market allowing the measurement of the TOC such as TOC-meters provided by Shimadzu (SSM-5000A), Horiba (EMIA-V), LECO or the RockEval6. The principle of the measurement is to oxidize a weighed ground-up or powdered sample such as drill cuttings and/or soil by gradually heating up the sample, first to a lower temperature to oxidize any organic molecules from light volatile carbon compounds to kerogen present, and then to a higher temperature to decompose carbonate compounds present in the sample. The combustion reaction will produce mainly carbon dioxide (CO2), carbon monoxide (CO), but also other compounds such water (H2O), nitrogen oxide and nitrogen dioxide (NO and NO2), and sulfide oxide (SO2). A suitable detector (e.g., IR, TCD) may measure the CO2 produced as a function of temperature, and the measured CO2 is related to a percentage of carbon compounds in the sample.
Two common methods are applied to calculate the TOC: a method by difference (indirect) and a direct calculation. The method by difference includes determining the total carbon content (TC) and the total inorganic content (TIC), followed by the calculation of the difference to determine the TOC (TOC=TC−TIC). The direct calculation involves analyzing a sample that is already free of carbonates, in which case the determined TC is equal to the TOC.
It is well known to determine the TOC of drill cuttings. However, drill cuttings are frequently contaminated with hydrocarbons or other organic compounds (e.g., from drilling fluids) which could introduce errors into the reading of the TOC, typically an over-estimation, in both the direct and indirect TOC calculation methods. In particular, due to the nature and the composition of some drilling fluids (e.g., oil-based drilling mud or drilling mud additives), field drill cutting samples contain extra carbon compounds that do not belong to the original organic matter present in the drill cuttings. As drilling fluids are complex mixtures including many carbon-containing compounds, contaminates including drilling fluids are generally removed before analyzing drill cuttings for TOC such as via an appropriate thermal cleaning procedure (e.g., gradually heating the drill cuttings to a relatively low temperature to remove the contaminants) or via any chemical cleaning procedure (e.g., chemically removing the contaminants using an appropriate solvent). Moreover, to perform a precise measurement an acidic treatment is also recommended to remove the carbonates from the samples, by this way only the organic carbons are remaining to be oxidized. Existing cleaning procedures include multiple chemical processes which are often time-consuming and skill-intensive, and involve the use of cleaning agents and/or acidic reactants that are expensive, corrosive, and difficult to transport, handle, and store. If the cleaning procedure does not completely remove the contaminants, a bias will likely occur on the TOO value.
To minimize the chances of a bias (i.e., underestimation or overestimation) on the TOC values in drill cuttings, determination of the TOC of drill cuttings is performed at the laboratory level where the appropriate chemical and/or thermal cleaning treatment of the drill cuttings is carried out prior to measuring the TOC. Whatever the cleaning procedure used, cleaning, processing, and analysis of drill cutting samples according to existing methods effectively precludes providing a real-time analysis of TOC in drill cuttings in the field, because existing methods employ lengthy and complicated cleaning, preparation, testing, and interpretation procedures which are expensive, time-consuming, and skill-intensive.